1. Field of the Invention
This invention relates to electrostatic powder spray guns, and more particularly to a gun having a rotating member at the powder outlet for distributing the powder in a uniform spray pattern.
2. Description of the Prior Art
In electrostatic powder painting, dry paint particles are fluidized in a powder hopper and pumped with conveying air through a hose to one or more spray guns which spray the powder onto a product to be coated. The spray guns impart a charge to the powder particles, typically with a high voltage charging electrode. When the powder particles are sprayed from the front of the gun, they are electrostatically attracted to the product to be painted which is generally electrically grounded and which may be suspended from an overhead conveyer or otherwise carried in a spray booth. Once these charged powder particles are deposited onto the product, they adhere there by electrostatic attraction until they are conveyed into an oven where they are melted to flow together to form a continuous coating on the product. Powder coating technology offers significant economic and environmental advantages over solvent-based liquid painting operations. Recently, powder coating materials have been developed which enable automobile manufacturers to employ powder coating applications on vehicle bodies in order to accommodate ever-growing environmental regulations.
The most recently developed powders for automotive finishes are typically of fine particle size, with the particles size of 20 microns or less, in order to enhance the smoothness and appearance of the finished coating. This small size, coupled with the chemistry of the powder material, creates a tendency for the individual particles to agglomerate or stick together, forming large masses of powder which are capable of generating surface defects. These agglomerates are generated as a result of particle segregation as the powder is in motion during the fluidizing, material conveying and application phases of the application process. If these agglomerated masses make it through the application system without breaking up, they form small visible bumps on the part being coated. These bumps are sometimes known as "spits" or "powder balls." Once the finished surface passes through the oven, these bumps become visible defects which must be sanded smooth before the final top coating. In large numbers, they become labor intensive and time-consuming, even causing stoppage of the finishing line.
It is believed that powder spray guns with rotating distributors at the powder outlet provide improved and more uniform spray patterns and other benefits. The designs of many powder spray guns of this type have similarities to liquid spray guns that have rotating atomizers at the fluid outlet. Examples of liquid spray guns of this type are shown in U.S. Pat. Nos. 4,887,770 and 5,346,139. The rotating atomizers in liquid spray guns rotate at very high speeds, with a typical speed of such spray guns being around 20,000-50,000 rpm. These high speeds are necessary because the atomizers must atomize the liquid coating material, and the atomization is best achieved at these speeds. The guns are not generally designed to be capable of slower speeds, because slower speeds would not effectively atomize the liquid.
An example of a powder spray gun having design similar to one of these liquid spray guns is shown in U.S. Pat. No. 5,353,995, in which a powder spray gun has a rotating distributor or deflector at the powder outlet and in which the distributor is turned by means of a turbine located in the gun. The adoption of the designs of liquid spray guns having rotary atomizers to the design of powder spray guns having rotary distributors results in several problems.
One of these problems involves the use of the high-speed air turbine motor as the distributor driver. If the distributor in a powder spray gun rotates at speeds as high as 30,000-50,000 rpm, the powder particles will acquire a kinetic energy which will turn to heat as the powder particles contact the distributor, causing the powder to fuse onto the rotating distributor. The problem of powder fusing has become more acute as new powders have been developed which are finer in size and which are susceptible to fusing more easily.
In addition to the problem of powder fusing, some powder spray guns having rotary distributors which are currently commercially available have developed a reputation for being prone to creating agglomerates and "powder balls" or "spits." This problem results from the design of the powder path within the spray gun as well as the high rotational speed of the distributor.
Some of recently developed powders which are more prone to building up on the rotary distributor due to impact fusion, are also more likely to build up elsewhere in the powder flow path. Unlike liquids, powder tends to accumulate at various locations in the flow path, and such powder accumulations can have various adverse effects. The built-up powder can eventually break loose and become deposited on the part being coated. Powder can also accumulate in areas around the bearings of the rotating components, which can cause excessive wear on the components and impede the free rotation of the components.
Further problems arise where rotating members engage stationary members along the powder flow path, since a rotary seal is required at this point of engagement to prevent powder from entering between the rotating and stationary members and can eventually enter into the bearings. If enough powder enters the bearings, heat created by the friction of the bearings can cause the powder to cure, creating drag which further slows the rotating members, and which can even cause lockup in extreme cases. Conventional seals, such as lip seals, O-rings, wiper rings and U-cups, could be used to exclude powder from the bearings. However, these seals when conventionally mounted must be squeezed against the rotating surface in order to work properly. The squeezing force is objectionable because frictional drag is thus created which cannot be overcome without inordinately increasing the size of the drive train or the size and power requirements of the motor, and increasing the power would lead to increased heat dissipation problems. Also, the heat created by frictional drag would likely cause residual powder to cure on the seal, on the rotating members and on adjacent surfaces. In addition, these conventional seals are designed to operate against metal surfaces, usually hardened steel, and would be unsatisfactory where the rotating members and bearings are made of plastic material because of electrostatic charging concerns. Plastic materials do not approach the hardness of steel, and the squeezing force applied to conventional seals would cause wear of the plastic rotating members at the point of contact.